The present invention relates to a resin-molded semiconductor device in which a semiconductor chip and signal-connecting leads to be connected to the chip are encapsulated with a resin encapsulant, a method for manufacturing such a device, and a lead frame suitable for manufacturing the resin-molded semiconductor device. In particular, the present invention relates to an improved device with a reduced thickness.
In recent years, in order to catch up with rapidly advancing downsizing of electronic appliances, it has become increasingly necessary to mount semiconductor components with higher and higher density. Correspondingly, sizes and thicknesses of semiconductor components have also been noticeably reduced.
Hereinafter, a conventional resin-molded semiconductor device will be described.
FIG. 20 is a cross-sectional view of a conventional resin-molded semiconductor device. As shown in FIG. 20, this semiconductor device includes external electrodes on its back surface.
The semiconductor device further includes a lead frame consisting of: inner leads 101; a die pad 102; and support leads (not shown) for supporting the die pad 102. A semiconductor chip 104 is bonded onto the die pad 102 with an adhesive, and electrode pads (not shown) of the chip 104 are electrically connected to the inner leads 101 with metal fine wires 105. And the die pad 102, semiconductor chip 104, part of the inner leads 101, support leads and metal fine wires 105 are encapsulated with a resin encapsulant 106. In this structure, no resin encapsulant 106 exists on the back surface of the inner leads 101. In other words, the respective back surfaces of the inner leads 101 are exposed and the respective lower parts of the inner leads 101, including the exposed back surfaces thereof, are used as external electrodes 107. To improve the adhesion between the resin encapsulant 106 and the inner leads 101 or the die pad 102, the side faces of the leads 101 and the pad 102 are formed like a taper with an upwardly increasing thickness, not to extend perpendicularly to their upper and lower surfaces.
In such a resin-molded semiconductor device, the respective back surfaces of the resin encapsulant 106 and the die pad 102 are both located on the same plane. Stated otherwise, the back surface of the lead frame is not substantially encapsulated. Accordingly, the thickness of such a semiconductor device is thinner than usual.
A resin-molded semiconductor device having such a structure as that shown in FIG. 20 i s manufactured in the following manner. First, a lead frame including inner leads 101 and a die pad 102 is prepared and then wrought mechanically or chemically to shape the side faces of the lead frame like a taper. Next , a semiconductor chip 104 is bonded onto the die pad 102 of the lead frame prepared, and is electrically connected to the inner leads 101 with metal fine wires 105. As the metal fine wires 105, aluminum (Al) or gold (Au) wires may be appropriately used, for example. Then, the die pad 102, semiconductor chip 104, inner leads 101, support leads and metal fine wires 105 are encapsulated with a resin encapsulant 106. In this case, the lead frame, on which the semiconductor chip 104 has been bonded, is introduced into a die assembly and transfer-molded. In particular, resin molding is performed with the back surface of the lead frame in contact with an upper or lower die of the die assembly. Finally, parts of outer leads, protruding outward from the resin encapsulant 106, are cut off, thereby completing a resin-molded semiconductor device.
Although a conventional resin-molded semiconductor device of this type has a reduced thickness, the device has the following problems.
Firstly, a resin encapsulant covers the upper and side faces of a die pad, but does not exist on the back surface thereof. Accordingly, the resin encapsulant cannot hold the die pad and the semiconductor chip so strong as that of other thicker devices, resulting in deterioration in reliability of the device.
Secondly, stress applied by the resin encapsulant or stress applied after assembling might have unwanted effects on a semiconductor chip or the resin encapsulant might possibly crack. Particularly when moisture penetrates between the die pad and the resin encapsulant, the adhesion therebetween decreases to a noticeable degree or considerable cracking is created. As a result, the reliability of the device further deteriorates.
Thirdly, although a resin-molded semiconductor device can be generally mounted accurately on a motherboard to be self-aligned with its desired position using the tension of solder, the time taken to settle the self alignment is still to be shortened and the mounting accuracy is still to be improved.
Fourthly, if part of a resin encapsulant sticks out of the back surface of a die pad in bonding the die pad and a motherboard (i.e., if so-called resin burr exists), then desired characteristics might not be attained in terms of heat radiation, for example. This is because the die pad cannot be in satisfactory contact with a heat-radiating pad in such a case. Such resin burr can be removed by using water jet or the like. However, such a process is not just troublesome, but causes additional problems. Specifically, if a water jet process is carried out, then a nickel, palladium or gold plated layer might peel off and impurity might deposit on the exposed parts. Accordingly, such parts exposed on the resin encapsulant should be plated once again after the encapsulation. As a result, work efficiency and reliability of the device might possibly deteriorate.